1. Field of the Invention
The present invention relates to a method for measuring stress exerted on an optical fiber, and in particular to a method for measuring stresses exerted on an optical fiber during manufacturing of the fiber or of a cabled fiber.
2. Technical Background
Numerous forms of fiber optic sensors have been developed to monitor parameters in various systems and processes, including the Fabry-Perot Interferometer, the Bragg Grating, the Mach-Zehnder Interferometer, and the Michelson Interferometer, to name but a few. These fiber optic sensors are used in a wide variety of applications, including use as strain gauges, dynamic pressure sensors, bearing condition sensors, non-contact proximity sensors, and temperature sensors. In each of these applications, the fiber optic sensor is fixedly attached to the system to be monitored, and usually is encased within a housing or rigid structure that is fixedly attached to the system to communicate parameter changes in the system to the fiber optic sensor.
As strain gauges, fiber optic sensors have been used to monitor dynamic strain. In such applications, the fiber optic sensor is imbedded within a material that is attached to a component of a structure such that the strain within the component may be monitored. Applications of fiber optic strain gauges have typically included civil structures such as dams, buildings, and bridges.
As dynamic pressure sensors, fiber optic sensors have been used in a variety of applications including the monitoring of performance of internal combustion engines, as well as monitoring the performance of compressors and pumps. When used to monitor the performance of an internal combustion engine, the fiber optic sensor is typically placed within a housing mated with a cylinder of the engine. The housing typically has a metal diaphragm that is attached to one end of the fiber optic sensor. Pressures exerted on the diaphragm are transferred to the fiber optic sensor, thereby changing the overall length of the sensor and allowing measurement of continuous real-time in cylinder pressures permitting improved engine control, providing preventive maintenance data, and predictive emissions monitoring. When used to monitor the performance of compressors and pumps, the fiber optic sensor is imbedded within an aluminum alloy rod, or similar metal, by an encasing process. The aluminum rod encasing the fiber optic sensor is then placed within a metal housing having a diaphragm similar to that described above in relation to engine monitoring. By placing the diaphragm in contact with the fluid being transferred by the compressor and/or pump, measurements of cavitation, flow instability, and surge detection are possible, thereby reducing the risk of catastrophic mechanical failure.
As bearing condition sensors, fiber optic sensors are used to monitor the condition of bearing or rotor imbalance. Typically, the fiber optic sensor is encased within a housing that includes a deformable diaphragm. The fiber optic sensor is in contact with the diaphragm which is, in turn, in contact with the outer race of a bearing, thereby allowing for the transfer of any vibrations between the associated bearings and the outer race to the fiber optic sensor.
In non-contact proximity sensors, fiber optic sensors are used to measure shaft vibration, rotor trust position, shaft rotational speed, as well as rotor imbalance and misalignment. In these applications, the fiber optic sensor is encased within a steel rod having a magnet attached to an end thereof. The steel rod encasing the optical fiber and the magnet are positioned within a stationary housing. The housing is then located such that the magnet is in close proximity to the rotating shaft to be monitored. Imbalances in the shaft cause the magnet to move which motion is transferred to the optical sensor for monitoring of the position or condition of the shaft.
As temperature sensors, fiber optic sensors are typically inserted into areas desired to be monitored, or imbedded into cast parts, thereby allowing the direct measurement of temperatures therein.
Typically, fiber optic sensors have been used to monitor systems that allow for stationary or fixed placement of the sensor within the system. The construction of these sensors have made it difficult if not impossible to monitor processes, systems, or machines that require the optical fiber and the associated fiber optic sensor to be moved throughout the system being monitored. Further, these systems typically require the fiber optic sensor to be cast within a part or structure to be monitored, or placed within a housing that is attached directly to the system to be monitored, thereby adding to the size and cost associated with the monitor system.
The manufacturing procedures and processing of optical fibers and fiber optic cables are numerous and varied. Many of these processes include placing a stress on the optical fiber or fibers being processed. These stresses when applied over time, however short, result in sub-critical growth of the pre-existing flaws located within the optical fibers, thereby decreasing the overall strength of the optical fiber. In certain applications, it is important that the optical fiber, or bundle of fibers, has sufficient strength to withstand loads place thereon without damaging the optical fiber or overall fiber optic cable. As a result, reliability models are created to estimate the strength of the fiber and the associated fiber optic cables after the processing and manufacturing. Reliability models for optical fibers are based on three things: the size distribution of flaws or cracks within the fiber; fatigue crack growth parameters; and the stress-time profile which the fiber experiences during processing. High-stress processing events may result in degradation of the fiber strength. Until now, direct measurements of the stresses exerted on an optical fiber during high-speed processing has not been possible, and, as a result, the stress-time profile of optical fiber has been an assumed quantity.
The ability to collect real-time measurements of the stresses exerted on an optical fiber during processing and cable manufacturing would be valuable for reliability analysis and modeling, process and equipment design, trouble-shooting of manufacturing lines, as well as fiber and cable installation.